window.onload = function(){
    // 获取随机数
function rand(m,n){
    return Math.ceil(Math.random()* (n-m+1)) + m -1
}
for(let i=0;i<50;i++){
    const i = document.createElement("i");
    i.style.height = rand(30,80) + 'px';
    i.style.left = rand(1,99) + 'vw';
    i.style.animationDuration = rand(5,30) / 10 +"s"; 
    document.querySelector(".Box").appendChild(i);
}
}